$ perl -wle 'print reverse(q{hello})'
hello
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Because print takes a LIST and hence provides a list context to reverse which triggers the list context behavior from the latter (and a 1 element list reversed is indistinguishable from the original 1 element list for obvious reasons). Stick a scalar in there after the print, and/or store the reverse into a scalar temporary variable, and you'll see the behavior you're expecting.

The cake is a lie.
The cake is a lie.
The cake is a lie.

Said code (and then some):

$\ = "\n";

print(my @a = reverse('hello'));  # hello (order or scalars reversed)
print(my $s = reverse('hello'));  # olleh (order of chars reversed)

print(reverse('hello'));          # hello (order or scalars reversed)
print(scalar(reverse('hello')));  # olleh (order of chars reversed)

sub f(@) { return @_; }
sub g($) { return @_; }

print(f(reverse('hello'));        # hello (order or scalars reversed)
print(g(reverse('hello')));       # olleh (order of chars reversed)
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Because you did not RTFM :)

reverse LIST
        In list context, returns a list value consisting of the elemen
+ts
        of LIST in the opposite order. In scalar context, concatenates
        the elements of LIST and returns a string value with all
        characters in the opposite order.

            print reverse <>;           # line tac, last line first

            undef $/;                   # for efficiency of <>
            print scalar reverse <>;    # character tac, last line tsr
+if

        Used without arguments in scalar context, reverse() reverses $
+_.

        This operator is also handy for inverting a hash, although the
+re
        are some caveats. If a value is duplicated in the original has
+h,
        only one of those can be represented as a key in the inverted
        hash. Also, this has to unwind one hash and build a whole new
        one, which may take some time on a large hash, such as from a
        DBM file.

            %by_name = reverse %by_address;     # Invert the hash
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